The invention relates to cellulose ether composition for use as a water retention agent, plasticizer and lubricant in the extrusion of inorganic compositions. In addition, the invention relates to the use of this cellulose ether composition in building materials and other mixtures, and also to a process for the extrusion of inorganic compositions using these additives. The use of this methylcellulose composition leads to improved processing properties, to a higher surface quality of the extruded body, and also to improved product properties of the extruded article.
The extrusion of inorganic compositions has been employed for many years; it is a process for shaping pasty inorganic mixtures into any desired profiles by pressing through a die. The articles thus obtained may be used in many ways, in particular in construction applications. Extruded inorganic compositions can have different constituents, those which are technically and economically important are, inter alia, extruded cement compositions. These contain in principle cement as binder, possibly also other binders, in addition aggregates (sands) and/or lightweight aggregates, possibly also constituents fibers and/or other additives, and also cellulose ethers, in particular methylcellulose, as water retention agents, plasticizers and lubricants.
Methylcellulose in the present context means all methyl-containing cellulose ethers such as methylcellulose, methylhydroxyethylcellulose, methylhydroxypropylcellulose, methylhydroxyethylhydroxypropylcellulose, methylethylhydroxyethylcellulose, methylhydroxyethylhydroxybutylcellulose, and also cellulose ethers which contain not only methyl groups but also longer-chain hydrophobic side chains. The amount of methylcellulose used is generally between 0.1 and 6%.
Whether a composition, from the aspect of economic efficiency, may be extruded and processed to produce high quality products with good marketability is determined by different parameters:
Processing parameters: the composition must be able to be discharged homogeneously from the die at the lowest possible pressure and also at the highest possible rate. In addition, after exit from the die, it should have the lowest possible deformation (broadening at the die, width expansion) transversely to the direction of discharge.
Product properties: the surface of the extruded composition plays a critical role. It should be crack-free and as smooth as possible. Good surface properties are a problem especially at relatively high extrusion temperatures which are customarily established after relatively long extrusion time. Elevated strengths or faster setting of the extruded products may be further desirable properties.
Correspondingly, the object underlying this invention is to find a composition that imparts to the extrudable composition improved surface quality and the physical properties.
WO 01/16048A1 (James Hardie) describes a mixture of viscosity-increasing agents (as such, inter alia, various cellulose ethers are named) and dispersants (as such, sulphonated melamin-formaldehyde resins are named, and also polycarboxylates, which are customarily termed superplasticizers) as additive in extruded cement compositions having the purpose of achieving a possibility for savings of cellulose ether by a synergy between the two components. In addition, advantages such as better surface, lower extrusion pressure and at times increased extrusion rate are reported. WO 01/16048A1, however, gives no indication to a person skilled in the art that the subsequently described inventive mixture of three components, namely cellulose ether, superplasticizer and defoamer in certain ratios, despite a set water requirement (regulated to the same consistency) of the extruded composition gives the six advantages mentioned below.
EP1266877A2 (Shin-Etsu) discloses a hydraulically binding mixture which, inter alia, comprises a thickener (cellulose ether) and an defoamer (containing polyether groups or polyether components). It is observed that the linear width broadening of the discharged composition at the die (“spring back”) is reduced. However, the mixtures according to EP 1266877 A2 give very stiff compositions having poor extrusion properties.
WO 03/024884 A1 (Shin-Etsu and NMB) describes mixtures of polycarboxylic acid copolymer and/or a salt thereof with water-soluble cellulose ethers and an defoamer. The mixing ratio between the first and second components, between polycarboxylic acid copolymer or salts thereof and the cellulose ether, is preferably 50:50 to 99:1. The high storage life of the liquid mixture of the three components is particularly emphasized.